1. The plasma renin activity (PRA) was measured in 76 diabetic patients who were attending an outpatients clinic. Of these patients 16 had untreated hypertension and 28 had diabetic complications, which ranged from microaneurysms to renal failure and blindness. 2. Compared with age- and sex-matched normotensive control subjects, both normotensive and hypertensive diabetic patients had significantly higher PRA (P < 0·001). 3. Hypertensive diabetic patients also showed a higher PRA than matched hypertensive control subjects (P < 0·005). There were no significant differences between diabetic patients with hypertension or complications compared with those without these features. 4. Although this elevation of PRA could be due to a change in another component of the renin—angiotensin system, hypersecretion of renin is the most likely explanation.